Method of manufacturing valve system for capacity control of a screw compressor

ABSTRACT

A lift valve communicating with a compression chamber of a variable capacity screw compressor is set forth including a valve housing, a piston reciprocally received within the valve housing, a shaft having a first end connected to the piston and a second end extending from the housing. The valve further includes a valve element connected to the second end of the shaft having a valve surface exposed to the compression chamber and a reciprocation mechanism for reciprocating the piston within the housing. The reciprocation mechanism includes a first pressure passage communicating with the housing adjacent a side of the piston, and a second pressure passage communicating with the housing adjacent and opposed side of the piston, wherein the valve surface is positively displaced toward and away from the compression chamber of the variable capacity screw compressor in response to the application of fluid pressure to at least one of the first and second pressure passages to vary the capacity of the screw compressor. Further, the lift valve is manufactured integral with the manufacturing of the compression chamber of the variable capacity screw compressor. This manufacturing process includes securing at least one lift valve to a housing of the variable capacity screw compressor in an operating position. Once secured to the housing, the shaft and consequently the valve element is fully extended from the valve housing and maintained in such position thus simultaneously machining an inner surface of the compression chamber and the valve surface such that the valve surface forms a continuation of the inner wall of the compression chamber when the variable capacity screw compressor is operating at full capacity.

This is a Divisional application of Ser. No. 08/346,244, filed Nov. 23,1994 now U.S. Pat. No. 5,556,271.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a valve system for controlling thecapacity of a screw compressor. Particularly, the present invention isdirected to double acting lift valves for controlling the capacity of ascrew compressor as well as the manufacture of such double acting liftvalves.

BACKGROUND OF THE INVENTION

Rotary screw compressors of the type set forth herein comprise tworotors mounted in a working space which is limited by two end walls anda barrel wall extending therebetween. The barrel wall necessarily takesthe shape of two intersecting cylinders, each housing one of the rotors.Each rotor is provided with helically extending lobes and grooves whichare intermeshed to form chevron shaped compression chambers. In thesechambers, a gaseous fluid is displaced and compressed from an inletchannel to an outlet channel by way of the screw compressor. Eachcompression chamber during a filling phase communicates with the inlet,during a compression phase undergoes a continued reduction in volume andduring a discharge phase commimicates with an outlet. A rotary screwcompressor of this type is disclosed in U.S. Pat. No. 4,435,139.

Rotary screw compressors of this kind are often provided with valves forregulating the built-in volume ratio for the capacity of the compressor.When continuous regulation is required, slide valves are often used,however, as with other regulation needs, it is sufficient to use liftvalves. Such lift valves are mounted in the barrel wall of thecompressor or may be mounted in one of the end walls and in this regard,normally in the high pressure end wall.

Several solutions for controlling the capacity of screw compressorsoperating at a constant number of rotations have been proposed. One suchsolution is disclosed in U.S. Pat. No. 5,108,269 issued Apr. 28, 1992.This solution provides radially positioned valves in the side wall ofthe barrel with the valves being opened so as to communicate theparticular compression chamber with either the inlet or outlet manifold.However, as will be discussed in greater detail hereinbelow, with suchvalves, compression losses due to leakage clearance valve and betweenthe valves and the rotors are experienced to the extent that fullcapacity cannot be realized.

Of the above noted solutions, the use of conventional slide type valveswhich constitute a portion of the barrel of the compressor has theadvantage of providing a wide control range and the possibility that ata constant working pressure ratio in the compressor a relativelyconstant built in pressure ratio within the greater part of the controlrange can be brought about by means of a suitable dimensioning of theaxial discharge port. The main disadvantage of slide valves is that theyare expensive to manufacture in that close tolerances and accuratecentering are required. Further, the actuating system which is normallya hydraulic system is also relatively expensive and complicated.

Another solution is to use a rotary type valve wherein the valves are incommunication with slots formed in the barrel through which gas isrecirculated to suction to create at partial loads. This valvearrangement has the advantage of being less expensive to manufacturethan conventional slide valve types, however, the capacity control isnot as accurate as with slide valve arrangements. Further, built-inpressure ratio drops with decreasing loads are experienced. Moreover,leakage is obtained across the slots along the rotor bores, particularlyat higher loads and at full loads. This shortcoming will be described ingreater detail hereinbelow with respect to FIG. 7b. Accordingly, it hasbeen determined that the use of lift valves achieves an economic balancebetween the need for accurate capacity control as well as the need forminimizing manufacturing costs and operating costs. Lift valves of thistype have been known and permit successive compression nodes within thebarrel to communicate with one another, thus, effectively reducing thecapacity of the compressor. One such valve is disclosed in U.S. Pat. No.4,453,900 issued Jun. 12, 1984. Further, such valves may communicate anoverlying compression node with a recirculation passage which returnspressurized fluid to the suction side of the compressor. However, it isnoted that the opening of the lift valve is directly dependent upon thecompression spring as well as the internal pressure of the compressor.However, the actuation of such valves is unreliable due to friction,corrosion and other environmental factors which often degradate thepositioning of this type of lift valve. Further, while the face of thevalve element takes on the approximate shape of the barrel, the valveelement is separately formed by casting or other process withinpredetermined tolerances. In order to economically manufacture suchvalve elements, the tolerances must be some what relaxed which mayresult in the leakage of pressurized fluid between compression chambersthereby degrading the efficiency of the compressor.

Clearly there is a need for an accurately controlled and inexpensivelymanufactured valve system for controlling the capacity of a oil floodedrotary screw type compressor. Such a valve system to include a pluralityof serially positioned lift valves which may be readily manufacturedwithin a zero tolerance, with each when opened reducing the capacity ofthe compressor a predetermined mount.

SUMMARY OF THE INVENTION

A primary object of the present invention is to overcome theaforementioned shortcomings associated with known valve systems.

Another object of the present invention is to provide a series of liftvalves for effectively controlling the capacity of a screw compressor.

Yet another object of the present invention is to provide a series ofdouble acting lift valves for accurately controlling the position of thelift valve and thus the capacity of a screw compressor.

A further object of the present invention is to ensure reliableoperation of the double acting lift valves by providing a two way shaftseal about an exposed end of the valve for preventing leakage from thevalve and oil leakage into such valve.

An even further object of the present invention is to provide a seriesof lift valves wherein operating losses due to leakage about the valveare minimized while assembly costs are reduced.

A further object of the present invention is to provide a series ofdouble acting lift valves for controlling the capacity of a screwcompressor wherein a surface of each valve which is exposed to acompression chamber of the screw compressor forms an effectivecontinuation of a surface of the compression chamber of the screwcompressor.

Yet another object of the present invention is to machine the surface ofeach valve simultaneously with the machining of the surface of thecompression chamber of the screw compressor in order to reducemanufacturing cost as well as operating losses.

A further object of the present invention is to positively andaccurately axially position the surface of each valve during themachining of the surface of the operating chamber of the screwcompressor.

An even further object of the present invention is to maintain theradial positioning of the surface of each valve during the machining ofthe surface of the compression chamber as well as during the operationof the screw compressor.

Yet another object of the present invention is to provide a series oflift valves wherein each lift valve housing is a single cast unitthereby minimizing leakages associated with related valves and reducingassembly costs.

These as well as additional objects of the present invention areachieved by providing a series of lift valves communicating with acompression chamber of a variable capacity screw compressor with eachvalve including a valve housing, a piston reciprocally received withinthe valve housing, a shaft having a first end connected to the pistonand a second end extending from the housing. Each valve further includesa valve element connected to the second end of the shaft having a valvesurface exposed to the compression chamber and a reciprocation mechanismfor reciprocating the piston within the housing. The reciprocationmechanism including a first pressure passage communicating with thehousing adjacent a first side of the piston, and a second pressurepassage communicating with the housing adjacent an opposed side of thepiston, wherein the valve surface is positively displaced toward andaway from the compression chamber of the variable capacity screwcompressor in response to the application of fluid pressure to at leastone of the first and second pressure passages to vary the capacity ofthe screw compressor.

Additionally, the lift valve is manufactured integral with themanufacturing of the compression chamber of the variable capacity screwcompressor. This manufacturing process includes securing at least onelift valve to a barrel portion of the variable capacity screw compressorin an operating position. As mentioned above, the lift valve includes avalve housing, a shaft extending from and reciprocally received withinthe valve housing and a valve surface of a valve element secured to aremote end of the shaft. Once secured to the housing, the shaft andconsequently the valve element is fully extended from the valve housing.The process further includes maintaining the shaft in the fully extendedposition, and simultaneously machining an inner surface of thecompression chamber and the valve surface such that the valve surfaceforms a continuation of the inner wall of the compression chamber whenthe variable capacity screw compressor is operating at full capacity. Inthis manner, zero tolerance is evidenced between the valve structure andthe surface of the compression chamber.

These as well as additional advantages of the present invention willbecome apparent from the following detailed description of the inventionwhen read in light of the several figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of the screw type compressor andsupporting controls to which the present invention may be readilyadapted;

FIG. 2 is a perspective view of a partially cut away screw compressorincorporating valves in accordance with the present invention;

FIG. 3 is a block schematic view of the overall operation of the screwcompressor in accordance with the present invention;

FIG. 4 is a perspective view of a screw compressor housing incorporatingthe present invention;

FIG. 5 is an elevational view of the lift valve in accordance with thepresent invention;

FIG. 6 is a cross-sectional view of a lift valve in accordance with thepresent invention;

FIG. 7A is a cross-sectional view of the lift valve in accordance withthe present invention in operation in the screw compressor housing;

FIG. 7B is a cross-sectional view of a prior art spiral or turn valve inoperation in the screw compressor housing, and

FIG. 7C is a cross-sectional view of a prior art lift valve in operationin the screw compressor housing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to improved lift valves and improvedmethods for manufacturing such lift valves for rotary screw compressors.FIG. 1 is a diagrammatic view showing the compressor system 100 to whichthe present invention may be readily adapted. Compressor system 100preferably includes an improved oil-flooded rotary screw compressor 102and an electronic control system 104. In the preferred embodiment of theinvention, the compressor 102 as well as the several capacity reductionlift valves 322 (only one illustrated) are controlled in accordance withthe electronic control system described in a co-pending applicationentitled "System And Methods For Controlling Rotary Screw Compressors,"naming Steven D. Centers and Paul Burrell as inventors, filed Nov. 23,1994 and assigned to the same assignee as this application.

This related co-pending application is hereby incorporated in thepresent disclosure by reference, and constitutes the primary source ofdetailed disclosure of the electronic control system. However, thosefeatures of the control system that are most relevant to the operationof the present invention will be described briefly in enough detail tofacilitate use of the inventive capacity reduction lift valves.Referring again to FIG. 1, compressor 102 is powered by an electricmotor 214. Electronic control system 104 includes control housing 236(containing the main electronic control components of the system), andrelay housing 106 containing relays and switchgear for the system. Airend 314 of compressor 102 is connected to a air/lubricant reservoir 312,which provides air to service air output 346.

As referred to hereinabove, compressor 102 is provided with fourcapacity reduction lift valves. When actuated, each of these valves actsto effectively bypass a part of the compressor screw, reducing thecapacity compressor 102 by approximately 12.5%. Thus, by opening onevalve, a 12.5% reduction in output capacity is obtained, and by openingall four valves, capacity of the compressor is reduced by 50%.Intermediate levels of capacity of reduction, such as 25% and 37.5%, aresimilarly obtained by opening from one to four of the capacity reductionvalves. For clarity, only one capacity reduction valve, valve 322, isshown in FIG. 1. Each of the capacity reduction valves is a positivedouble acting air operated valve, and each is controlled by a four waysolenoid valve in response to signals from the electronic control system104. The four way solenoid valves for controlling the four capacityreduction lift valves are designated in the drawing as SV1, SV2, SV3,and SV4.

Compressor 102 has an inlet valve 336 controllable to vary the amount ofinlet air supplied to compressor 102. When inlet valve 336 is closed, noair is provided to compressor 102, so compressor 102 is "unloaded" andruns freely with minimal compression load. When inlet valve 336 is fullyopen, the compressor is "loaded" or provided with input air. Inlet valve336 can also be controlled to open partially in a "modulated" operatingmode, so that compressor 102 is only partially loaded. The operation ofinlet valve 336 is controlled by solenoid valves SV5 and SV7 whichrespond to signals from electronic control system 104. Valve SV5, whenactivated, closes inlet valve 336 and unloads compressor 102. Valve SV7,when activated, partially closes inlet valve 336 so that compressor 102is only partially loaded. Valve SV7 is connected to a proportionalregulator. Thus, when activated, valve SV7 provides closing pressurethrough the proportional regulator to inlet valve 336 that varies withthe pressure in reservoir 312. As system pressure is increased, theamount of closure of inlet valve 336 upon activation of valve SV7 isalso increased. Electronic control system 106 is also connected toblowdown valve SV6 which can be activated to release pressure from thesystem when unloaded and at shutdown.

Referring now to FIG. 2, the compressor 102 will now be described ingreater detail. Specifically, the compressor 102 is a constant velocityoil flooded rotary screw type compressor which is driven by an electricdrive motor 214 which drives the main shaft 6 which is supported bybearing assembles 8 and 10 which are housed in bearing housings 12 and14 respectively. Positioned at the end of the main shaft 6 is a positivedisplacement lubricant pump 16 for providing efficient lubricantinjection under all operating conditions. Secured to the main shaft 6 isa first rotor 18 while secured to a second rotary shaft (not shown)mounted parallel to shaft 6 includes a second rotor 20. The second shaftis similarly mounted in bearing housing 22. As discussed hereinabove,the screw type compressor includes an inlet valve 336 which controllablymoves between a closed position as illustrated in FIG. 2 and a fullyopened position when the screw compressor is operating at full capacity.Further, when the screw compressor is operating at less than fullcapacity, the inlet valve 336 may be positioned somewhere between afully opened and fully closed position or oscillated between suchpositions as described in the above-noted copending application.

As discussed hereinabove, lift valves 322, 324, 326 and 328 communicatewith the compression chamber 24 formed within the barrel 26 of thecompressor 102. As illustrated in FIG. 2, a bore 28 is provided in thebarrel 26 which may selectively provide communication betweencompression nodes and consequently reduces the capacity of thecompressor 102. Alternatively, bore 28 may communicate with passage 29in the barrel housing for returning pressurized fluid to the suctionside of the compressor. As discussed hereinabove, when in the opencondition, each of these valves act to effectively by-pass a part of thecompressor screw and thus reduce the capacity of the compressor byapproximately 12.5%. Accordingly, by opening all four valves, thecapacity of the compressor is reduced by 50%. It is the structure andprocess of manufacturing the lift valves 322, 324, 326 and 328 whichconstitute the essence of the present invention. Accordingly, thesevalves will be discussed in greater detail hereinbelow.

FIG. 3 is a block schematic diagram of air control line connections andair control equipment in accordance a preferred embodiment of theinvention. Again, this control system is discussed in detail in theabove-noted copending application and will be only briefly discussedherein. The air control equipment includes a control panel 302 having apressure switch 304, an air filter indicator switch 306, a line pressuretransducer 308, and a reservoir pressure transducer 310. Separatorscavenges 311 of reservoir 312 are connected to air end low pressurepoint 314 of compressor 102 through line filter orifices 316, sightgauges 318, and line filters 320.

The four way solenoid valves SV1 through SV4 are connected to controllift valves 322, 324, 326, and 328 respectively. Valves SV1 through SV4are preferably four-way positive action solenoid valves. An air supplyinput for valves SV1 through SV4 is connected to a pressurized airoutlet of reservoir 312 by way of pressure regulator 330 and automaticline filter 332. Pressure regulator 330 may be omitted if the compressorsystem 100 will not be operated above 125 psi full load pressure. ValvesSV1 through SV4 can also be connected by two lines to low pressure point333 below air filter 334, on inlet valve 336 which is installed on theair intake port of compressor 102. These two lines provide exhaust portsfor valves SV1 through SV4, for each direction of stroke of the valves.

The provision of double action lift valves 322, 324, 326, and 328 ratherthan single action lift valves provides a significant advantage in thecontext of compressor system 100. This feature will be described ingreater detail hereinbelow.

A reservoir air output 337 is connected to reservoir 312 to carry thecompressed air output of the compressor to the customer's service airpiping system, and thus to the equipment operating on the compressed airgenerated by compressor system 100. Air output 337 is connected throughan after cooler 339 to a minimum pressure check valve 341, the output ofwhich is connected to the customer's service air piping system atservice air output 346. Reservoir air output 337 is also connected to asolenoid operated blowdown valve SV6 which is connected to a muffler343. When blowdown valve SV6 is actuated, air pressure in reservoir 312is released to the environment through muffler 343.

The pressurized air outlet of reservoir 312 is connected by an air lineto reservoir pressure transducer 310, and a mechanical pressure gauge338 is connected to the same line next to reservoir 312. Similarly, apressurized air output of reservoir 312 is connected to an input ofautomatic line filter 340. The output of automatic line filter 340 isconnected to one air input side of shuffle valve 342 and to the input ofpressure regulator 344. The output of pressure regulator 344 isconnected to a non-common connection of three-way solenoid valve SV7.The other air input side of shuffle valve 342 is connected to thecustomer's service air at service air output 346 of compressor system100.

The output of shuffle valve 342 is connected to pressure switch 304 andto a non-common connection of three-way solenoid valve SV5. The commonconnection of three-way solenoid valve SV5 is connected to one air inputside of shuffle valve 350. The other air input side of shuffle valve 350is connected to the common connection of three-way solenoid valve SV7.The remaining non-common connection of each of three-way solenoid valvesSV5 and SV7 is open for exhaust. The output of shuffle valve 350 isconnected by an air pipe to the input of gauge/pressure regulator 354.The output of gauge/pressure regulator 354 is connected to the inletvalve 336 control side.

These particular air connection configurations and the use of three-wayvalves SV5 and SV7 are significant because they allow inlet valve 336 toreceive operating air pressure more quickly during startup, so thatinlet valve 336 can be immediately closed to provide an unloaded startupof compressor 102. At startup, there is no pressure in reservoir 312.There may, however, be pressure in the customer's service air line, dueto stored pressure in an external reservoir and/or because othercompressors are running to pressurize the service air line. It has beendetermined that when service air pressure is available, it isadvantageous to make use of this pressure for startup control during theperiod before reservoir 312 is pressurized.

At startup, the existence of pressure in the service air line and thelack of pressure in reservoir 312 will bias shuttle valve 342 to connectthe service air line to three-way solenoid valve SV5. Three-way solenoidvalve SV5 is then actuated to transmit the service air pressure toshuttle valve 350, while three-way solenoid valve SV7 is controlled toconnect its common connection to the exhaust end. The service airpressure biases shuttle valve 350 to connect the service air pressure tocontrol inlet valve 336. Valve SV5 is then actuated, which will unloadcompressor 102 prior to starting motor 214. In this way, compressorsystem 100 can be started without any loading, minimizing startup powerusage and transient currents. When sufficient pressure is available inreservoir 312, air from reservoir 312 is provided to bias shuttle valve342 toward three-way solenoid valve SV5, allowing transmission of thereservoir air to the inlet valve 336 control side.

Referring now to FIG. 4, the barrel portion 26 of the screw compressorhousing is illustrated in detail. The barrel portion 26 is formed bycasting and subsequently machined to receive the respective rotors. Thebarrel wall necessarily takes the shape of two intersecting cylinders,each housing one of the rotors 18 and 20. As discussed hereinabove withrespect to FIG. 2, lift valves 322, 324, 326 and 328 of which only liftvalve 322 is illustrated communicate with the compression chamber 24within the barrel 26 by way of bores 28. The double acting lift valve322 includes a mounting flange 323 which permits the double acting liftvalve 322 to be secured to the barrel 26 by way of bolts 325 (one ofwhich is shown). In order to assure proper alignment of the lift valvewith the barrel 26, opposed bolt holes 321 in flange 323 as well as thebarrel 26 are staggered. By doing so, the lift valve can only be mountedin one orientation. Also provided is a gasket 327 for providing a sealbetween the barrel 26 and mounting flange 323. The remaining lift valves324, 326 and 328 are similarly mounted to the barrel 26 in this manner.

In accordance with the present invention and in order to form a moreefficient screw compressor, each of the double action lift valves aresecured to the barrel 26 in a manner discussed with respect to FIG. 4and machined along with the machining of the surface 25 of compressionchamber 24 within the barrel 26. Referring to each of FIGS. 5, 6 and 7a,it can be noted that the surface 402 which is exposed within thecompression chamber 24 of the barrel 26 takes on a concave shape due toits machining along with the machining of the compression chamber 24 ofthe barrel 26.

Referring to FIG. 5, the double action lift valve 322 includes a housing410 which accommodates a piston (not shown) and piston stem 412. Formedintegral with the piston stem 412 is a valve element 414 which includesthe concave surface 402. Additionally, a flange 416 is provided forpositioning the valve against the barrel 26 when the valve is in thefully extended position as illustrated in FIG. 7a. Again, the doubleaction lift valve includes a mounting flange 323 which is cast with thehousing 410 for securing the valve in place. In order to seal bothpressurized air within the housing 410 as well as sealing out any oilwhich may leak past the flange 416, a two-way shaft seal 418 is securedto an end of the housing 410. The inner details of the lift valve 322will now be discussed in greater detail with respect to FIG. 6.

As can be seen from FIG. 6, the piston stem 412 is integrally formedwith a piston member 411 which is reciprocally received within thehousing 410. The piston stem 412 and piston member 411 may also beseparate units secured to one another in any known manner. Further, itshould be noted that the housing 410 is in the form of a one-piececylinder casting. With previous lift valves, the valve casing or housing410 is formed from multiple sections which are secured to one anotherusing sealing gaskets and the bolts. However, it has been determined bycasting a single piece housing, not only are previous leakage pointseliminated, the assembly time for assembling the lift valve is alsoreduced. Further, with the one-piece construction, the flange 323 aswell as bolt holes 321 can be so oriented that the lift valve 322 canonly be mounted on the barrel in a single orientation therebyeliminating incorrect installation of the lift valves if such valves areremoved for shipping or service as referred to hereinabove. Again, it iscritical that the lift valves be installed in the orientation in whichthey are initially manufactured such that the concave surface for 402 isproperly oriented within the compression chamber 324. Additionally, inorder to assure that the piston 411 and piston stem 412 do not changeorientations with respect to the housing 410 after manufacturing, asquare pin 414 is received within a square hole 416 formed in the piston411. In doing so, the square pin 414 will prohibit any rotation of thepiston 411 with respect to the housing 410. While the particularembodiment illustrated in FIG. 6 includes the square pin 414 and squarehole 416, any mechanism for maintaining the orientation of the piston411 with respect to the housing 410 may be utilized. The primary concernis to assure the proper orientation of the concave surface 402 withinthe compression chamber 324. Such an orientation may be maintained byany acceptable means.

When the lift valve 322 is assembled, two pressure chambers are formed,one being pressure chamber 418 between the end of the housing 410 andthe piston 411 the other being a second pressure chamber 426 formedbetween the piston member 411 and the two-way shaft seal 418. Again, asdescribed hereinabove, the two-way shaft seal 418 is provided in orderto seal in both directions, that is the two-way shaft seal 418 seals inpressurized air within the pressure chamber 426 and seals out any oilexternal to the valve.

As discussed hereinabove, each of the lift valves 322, 324, 326 and 328are actuated and de-actuated by way of four-way solenoid valves SV1,SV2, SV3 and SV4 respectively. That is, in order to manipulate thepiston 411 within the housing 410, pressurized air may be provided toeither one of pressure chambers 424 or 426 while the other of thepressure chambers are exhausted. That is, in order to force the valveelement 414 into the fully extended closed position, pressurized air isprovided to the pressure chamber 424 through passage 425 while thepressure chamber 426 is exhausted through passage 427. It should benoted that both passages 425 and 427 are positioned in a lower portionof the valve housing 410. This assures that condensation will beproperly drained from the chamber 424 and 426 respectively. Likewise,should it be desired to operate the screw compressor at less than fullcapacity, pressurized air is supplied to the pressure chamber 426through passage 427 of one or more of the lift valves while the pressurechamber 424 is exhausted through passage 425 in order to reciprocate thepiston 411 and consequently the valve element 414 to an open position.As discussed hereinabove, four way solenoid valves SV1, SV2, SV3 and SV4are controlled to selectively pressurize and exhaust pressure chambers424 and 426 in response to a demand placed on the compressor system. Inorder to isolate the pressure chambers 424 and 426 from one another,piston 411 is provided with seals 428 and 430. Also, seal 432 isprovided in the two-way shaft seal which is secured to an open end ofthe housing 410.

As discussed hereinabove, the surface of the valve element 414 ismachined integral with the machining of the surface 25 of thecompression chamber 24 of the barrel 26. That is, during the finalmachining of the compressor chamber side walls 25, in order to form therequisite tolerance between the rotors and such side wall, each of thelift valves 322, 324, 326 and 328 are positioned in their operatingposition secured to the barrel 26. In this regard, the piston 411 andconsequently the valve element 414 must be fully extended and maintainedin the fully extended position throughout the machining process andparticularly when the surface 402 itself is being machined. In order todo so, the pressure chamber 424 is filled with pressurized hydraulicfluid or oil which assures that the valve element 414 will remain in itsfully extended position assuming such fluid to be incompressible.Accordingly, once the lift valves 322, 324, 326 and 328 are secured tothe barrel 426, pressure chamber 424 is filled with an incompressiblefluid at which time the final machining of the wall 25 of thecompression chamber 24 is carried out. In doing so, the surface 402 ofthe valve element 414 exactly matches and forms a continuation of thewall 25 of the compression chamber 24 which minimizes any leakage aroundthe rotor as the rotor passes over the surface 402.

Referring now to FIGS. 7A through 7C, the distinct advantage of thepresent invention over prior art valving systems will become clearlyapparent. The present invention is illustrated in FIG. 7a wherein thevalve element 414 is positioned in its fully extended position. As canbe seen from FIG. 7A, the surface 402 of the valve element 414 forms acontinuation of the surface 25 of the compression chamber 24 of barrel26. Accordingly, as rotor 18 rotates past the valve element 114, thereis no leakage between the surface 402 of the valve element 414 and therotor 18. This is achieved because the surface 402 is machined integralwith the surface 25 of the barrel 26. Further, the positioning of thevalve element 414 is assured due to the positive displacement of thepiston within the double acting lift valve. While the aforementionedprior art devices illustrate lift valves having concave surfaces, suchlift valves are formed by way of a separate manufacturing process andsubsequently positioned within the compressor housing. Accordingly,these lift valve surfaces are manufactured to within predeterminedtolerance, however, such manufacturing process cannot practicallyduplicate the curvature of the compression chamber surface 25 and thusleakage by the rotor may still exist in such systems.

Referring to FIG. 7B, clearly when using a turn and spiral valvevariable capacity design, numerous ports 50 are provided near the bottomcenter line of the barrel 26'. As discussed hereinabove, these ports areas deep as the housing material is thick and consequently air in thehigher pressure compression pocket blows around the tips of the rotors18 and 20 as they pass these ports. Clearly, the efficiency of thedevice is significantly reduced and full capacity cannot be achieved.

The poppet type valve illustrated in FIG. 7C includes a planar surface52 on the valve element 54 which also allows blow by around the rotor 18resulting in a reduction in the efficiency of the system. Further, sucha poppet type valve relies on a single acting piston to close thevalves, thereby relying on the internal air pressure and/or a springforce to move, the valve to the open position. Often times, the openingpressure may be low and consequently these valve designs may stick oroperate erratically, again failing to provide the user with the maximumsavings under part load conditions.

Clearly, it can be seen that by utilizing double acting lift valveshaving a single valve cylinder casting with a valve element which ismachined in conjunction with the machining of the compression chamberwall provides an advantageous capacity control system wherein thecompressor can realize 100% efficiency when the double acting liftvalves are in the closed position and which may accrately control thecapacity reduction as desired.

While the present invention has been described with reference toreferred embodiments, it will be appreciated by those skilled in the artthat the invention may be practiced otherwise than as specificallydescribed herein without departing from the spirit and scope of theinvention. Therefore, it will be understood that the spirit and scope ofthe invention be limited only by the appended claims.

What is claimed:
 1. A method of manufacturing a lift valve for use in avariable capacity screw compressor comprising the steps of:securing atleast one lift valve to a housing of the variable capacity screwcompressor in an operating position, said lift valve including a valvehousing, a shaft extending from and reciprocally received within saidvalve housing and a valve face seethed to a remote end of said shaft;fully extending said shaft from said valve housing; maintaining saidshaft in said fully extended position, and simultaneously machining aninner surface of said compressor housing and said valve face; whereinsaid valve face forms a continuation of said compressor housing whensaid variable capacity screw compressor is operating at full capacity.2. The method as defined in claim 1, further comprising the step ofmaintaining an angular orientation of said valve face with respect tosaid valve housing during said machining step.
 3. The method as definedin claim 1, wherein said step of maintaining said shaft in said fullyextended position includes pressurizing at least one chamber within saidvalve housing.
 4. The method as defined in claim 3, wherein said chamberis pressurized with a substantially incompressible fluid.
 5. The methodas defined in claim 3, wherein said incompressible fluid is oil.
 6. Themethod as defined in claim 1, wherein a plurality of lift valves aresecured to said housing prior to said step of machining said innersurface of said compressor housing.